This invention relates generally to radiation therapy equipment for the treatment of tumors or the like and specifically to a radiation therapy machine allowing individual real-time control of the intensity of multiple rays within a treatment radiation beam.
Medical equipment for radiation therapy treats tumorous tissues with high energy radiation. The amount of radiation and its placement must be accurately controlled to ensure both that the tumor receives sufficient radiation to be destroyed and that the damage to the surrounding nontumorous tissue is minimized.
One highly accurate method of controlling the dose to a patient employs a radiation source that produces a radiation beam composed of many individual rays whose intensity may be individually controlled. This beam may be produced by a series of shutters, each controlling one ray, or by a single modulated ray moving across the patient to create the beam over an interval of time.
The origin of the rays moves over a range of angles about the patient and by properly selecting the ray intensities at different angles, complex regions within the patient may be accurately irradiated. U.S. Pat. Nos. 5,724,400; 5,673,300; 5,668,371; 5,661,773; 5,625,663; 5,548,627; 5,528,650; 5,442,675; 5,394,542; and 5,317,616, all assigned to the same assignee as the present application, and hereby incorporated by reference, describe the construction of a machine of this type and a method of calculating the necessary ray intensities as a function of angle. In the described machine, shutters switched between open and closed states each control the intensity of a corresponding individual ray, however other methods are also known of delivering rays of varying intensities including those using a single modulated and scanning ray and other such systems.
The promise of improved accuracy of such radiation therapy systems and their increased complexity make it desirable to have a means of verification of the correct operation of the shutters and thus the intensity of the rays irradiating the patient. A post-patient radiation monitor, commonly known as an xe2x80x9cexit detectorxe2x80x9d, may provide an approximate indication of the correct operation of the radiation therapy machine, but patient attenuation, scatter, and the overlap of rays at detector elements of the post-patient radiation detector prevent the direct observation of shutter operation in a post-patient image. Nevertheless, the sensitivity of the post-patient radiation monitor to attenuation of radiation by the patient allows it to be used for imaging of the patient for registration and verification of patient positioning or the like with the shutters fully open.
Indirect verification of intensity may be provided by cameras viewing the shutters or sensors mounted on the shutters to indicate their correct operation and movement. Information about shutter operation together provides a general indication of the fluence of the rays directed toward the patient provided profile of the radiation beam incident on the shutters is known and reliable.
Preferably and as taught in U.S. Pat. No. 5,394,452 cited above, a pre-patient radiation monitor may be used to make direct measurement of the intensity of the rays. The pre-patient monitor provides a measurement of fluence received by the patient but is cumbersome and unlike a post-patient radiation monitor cannot be used to provide information about patient positioning or imaging.
The present inventors have recognized that a post-patient monitor alone may in fact be used to verify operation of the shutter system during radiation therapy on a particular patient if the response of the patient attenuation and ray paths can be accurately modeled. The model can then be inverted and applied to the detected radiation exiting the patient to give an indication of the entrance fluence of the individual rays or other related physical quantities such as energy. The entrance fluence together with some information about the patient, from a tomogram of the like, can be used to deduce the dose delivered to the patient.
The model may be constructed from a known geometry of the radiation therapy machine and estimated properties of the patient or standard patient, or properties of the patient as deduced from a pre-treatment tomogram or from an experimental or theoretically derived database, or may be derived by a selective excitation of different shutters and measurements of the received signals immediately prior to or during the radiation treatment session. The radiation used in such measurements may be xe2x80x9cborrowedxe2x80x9d from the treatment plan itself so as to leave the total dose to the patient unaffected.
Specifically, the invention provides a method of verifying operation of a radiotherapy radiation source in a radiotherapy machine operable to produce multiple rays of radiation having controllable fluence. The rays are directed across a patient volume to a detector of multiple detector elements which provide detector signals at spatially separate points, each detector element receiving radiation from multiple rays. The steps include receiving a treatment plan to operate the radiation source to produce a set of rays of predetermined fluence and operating the radiotherapy machine according the received treatment plan. Concurrently with the radiation therapy, detector signals are measured and processed using a model of the expected attenuation of each ray passing from the radiation source through the patient volume to the detector to deduce a measured fluence of each of the set of rays. Finally, the measured fluence is compared to the predetermined fluence of each ray to verify operation of the radiation source or to deduce the dose applied to the patient.
Thus it is one object of the invention to provide a radiation therapy system requiring only a single post-patient radiation monitor. Because the radiation monitor is not interposed between the patient and the radiation source, it may be in part or wholly absorbent providing greater flexibility in design. Unlike a pre-patient monitor, the post-patient monitor may be used for patient imaging and positioning tasks.
The model may be based on known geometry of the radiation therapy system optionally augmented by tomographic information about the patient. Alternatively, the model may be produced from a series of measurements of the patient or a phantom in which the radiation source is operated to produce a predetermined sequence of test fluences for the set of rays and those fluences used to produce a model. The data may be stored in a database and optionally sets of data combined to produce data for a standard patient.
Thus it is another object of the invention to provide a verification system that accurately accounts for modification of the beam as it passes through the patient.
The predetermined sequence of test fluences for the set of rays may provide a unit fluence for one ray at a time to create an impulse response of the patient/radiation source.
Thus it is another object of the invention to provide a simple method of characterizing the patient and the radiation path such as may be used for verification of radiation source output and/or dose by a post-patient radiation detector.
Multiple sets of rays may be simultaneously energized to the unit fluence in the predetermined sequence provided the rays do not overlap at the detector or overlap minimally.
Thus it is another object of the invention to provide a rapid method of acquiring the necessary data for the model used for verification.
The radiation treatment plan may be modified to subtract the fluences of the predetermined sequence of test fluences thereby leaving the dose to the patient unincreased.
Thus it is another object of the invention to provide extremely accurate characterization of the patient using the actual radiation used for treatment while avoiding the penalty of increased dose to the patient.
The predetermined sequence may provide a unit fluence only for rays actually used in the radiation treatment plan. Estimates may be made for those rays not so measured.
Thus it is another object of the invention to provide accurate modeling of the patient only for rays whose dose may be xe2x80x9cborrowedxe2x80x9d from other portions of the radiation plan preventing the increase of total dose to the patient.
It is another object of the invention, therefore, to provide some information for verification of rays that are not expected to be used in the radiation therapy system in order to track possible errors in which they would be open.
The treatment plan may include multiple sessions and the method may provide for correcting of the treatment plan to account for deviations between the measured fluence and the predetermined fluence for subsequent sessions.
Thus it is another object of the invention to accommodate minor errors in treatment through corrective action on subsequent treatments.
The foregoing and other objects and advantages of the invention will appear from the following description. In the description, reference is made to the accompanying drawings which form a part hereof and in which there is shown by way of illustration a preferred embodiment of the invention. Such embodiment does not necessary represent the full scope of the invention, however, and reference must be made to the claims herein for interpreting the scope of the invention.